In a fuel cell system having a number of fuel cells, electrical energy and heat are generated by hydrogen (H2) and oxygen (O2) being combined in an electrochemical reaction. For this purpose, hydrogen and oxygen are fed to the fuel cells, either in pure form or as fuel gas with a hydrogen content and as air. The type of operating gases which are fed to the fuel cell is mainly dependent on the operating environment in which the fuel cell system is operated. For example, if the fuel cell system is operated in a hermetically sealed space, it should be ensured that the fuel cell system produces the minimum possible amount of exhaust gases.
Since these exhaust gases will be unable to leave the hermetically sealed space, they have to be collected and possibly compressed. Therefore, a fuel cell system of this type is operated with pure oxygen and pure hydrogen, with a view to these operating gases, when the fuel cell system is operating, reacting to form water (H2O) substantially without residues, so that the fuel cell system produces scarcely any exhaust gases.
The output produced by a fuel cell during operation is dependent, inter alia, on the quantity of operating gases which flow into the fuel cell. If the flow of operating gases into the fuel cell is low, the power output by the fuel cell is also low. As the supply of fuel gas increases, it is also possible for the output of the fuel cell to rise until a saturation value is reached. To achieve a good level of output from the fuel cell, it is necessary for more operating gases to be passed through the fuel cell than the fuel cell actually consumes during operation. It is therefore inevitable that an efficiently operated fuel cell will produce exhaust gases, namely at least the operating gases which have flowed through the fuel cells without being used.
EP 0 596 366 B1 has disclosed a fuel cell block having a multiplicity of fuel cells, the fuel cells of the block being combined in a plurality of groups of cells. When this fuel cell block is operating, the operating gases flow in parallel into the fuel cells of the first group. The exhaust gases from the fuel cells belonging to this group, which contain operating gases, are introduced in parallel into the fuel cells of a second cell group, and the exhaust gases from the second cell group are fed to a third cell group. In this way, the operating gases are consumed ever more completely from group to group. However, it has been found that producing what is known as a cascaded fuel cell block of this type is very complex.